Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth

 As most of our foods are come from  plant and/or animal origin, which are to be consider those characteristics of plant and animal tissues that may be affect by the growth of microorganisms. The plants such vegetables and other edible plant and animals which  serve as food sources for us have all evolved mechanisms of defense against the invasion and proliferation of  many microorganisms, and many of these remain in effect in fresh foods. By taking those natural food  one can may be  effective to use of each or all in preventing or retarding the growth of pathogenic and spoilage organisms in the products that are derived from them.

 As most of our foods are come from  plant and/or animal origin, which are to be consider those characteristics of plant and animal tissues that may be affect by the growth of microorganisms. The plants such vegetables and other edible plant and animals which  serve as food sources for us have all evolved mechanisms of defense against the invasion and proliferation of  many microorganisms, and many of these remain in effect in fresh foods. By taking those natural food  one can may be  effective to use of each or all in preventing or retarding the growth of pathogenic and spoilage organisms in the products that are derived from them.


The parameters which are an inherent part of the an organisms are referred to as intrinsic parameter.The parameters present in substrates in which the microorganisms are growing, that are internal parts of the substrate are called as intrinsic parameters.

These parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures

1. pH:
 All the microorganisms have a minimal, maximal and optimal pH for their growth,development,  survival and activity of their enzymes. Growth of all may be microorganisms is affected by the pH of growth environments in food (growth medium) resulting large number of enzymes responsible for metabolism and growth. The Influenced of  pH of any  food not only has effect on their growth of microorganisms but also effect on their processing conditions. Food having acidic and basic  contents promotes growth of acid loving microorganisms such as yeasts, moulds and some acidophilic bacteria.

Mold are very small in size which can grow over a wider range of acidic pH than bacteria and yeast. Most of the argumentative yeasts can easily grow at pH of about 4.0 to 4.5, in fruit juices and acid food such as sauerkraut and pickles. A food with an acidic pH would tend to be more micro-biologically stable than neutral or alkaline food. Because of this restrictive level pH of food such as fruits, soft drinks, fermented milks, sauerkraut and pickles are very stable against bacterial spoilage..

Most of the bacteria, except acid fermenters are favored alkaline or neutral pH. Most of the bacteria preferred a pH range between 7.0-7.5 but some proteolytic bacteria can grow on food substrate with high pH. The buffer content in the food is important to maintain the stability against microbial spoilage.
Buffers permit an acid (or alkali) fermentation to go on longer with a greater yield of products and organisms. Vegetable juices have low buffering capacity permitting a decrease in pH with the production of only small amount of acid by the lactic acid bacteria during the early stage of sauerkraut and pickle fermentation. This helps to inhibit the growth of pectin hydrolyzing and proteolytic competing bacteria in food.
Food acidification by fermentation in home food preparations is the oldest practice man has been doing. It is due to production of organic acids in food by growth and fermentation of microorganisms such as lactic and acetic acid bacteria. The inhibitory properties of many of the organic acids such as citric acid, lactic acid, benzoic acid, propionic acid, sorbic acids, etc. can be used as effective acidulants or chemical preservatives against food spoilage bacteria.

2.Moisture Content
One of the oldest methods of preserving foods is drying or desiccation; precisely how this method came to be used is not known. The preservation of foods by drying is a direct consequence of removal or binding of moisture, without which microorganisms do not grow. It is now generally accepted that the water requirements of microorganisms should be described in terms of the water activity (aw) in the environment. This parameter is defined by the ratio of the water vapor pressure of food substrate to the vapor pressure of pure water at the same temperature: aw = p/po, where p is the vapor pressure of the solution and po is the vapor pressure of the solvent (usually water). This concept is related to relative humidity (RH) in the following way: RH = 100 × aw.13 Pure water has an aw of 1.00, a 22% NaCl solution (w/v) has an aw of 0.86, and a saturated solution of NaCl has an aw of 0.75 (Table 3–4). The water activity (aw) of most fresh foods is above 0.99. The minimum values reported for the growth of some microorganisms in foods is presented in Table 3–5 (see also Chapter 18). In general, bacteria require higher values of aw for growth than fungi, with Gram-negative bacteria having higher requirements than Gram positives. Most spoilage bacteria do not grow below aw = 0.91, whereas spoilage molds can grow as low as 0.80. With respect to food-poisoning bacteria, Staphylococcus aureus can grow as low as 0.86, whereas Clostridium botulinum does not grow below 0.94. Just as yeasts and molds grow over a wider pH range than bacteria, the same is true for aw. The lowest reported value for foodborne bacteria is 0.75 for halophiles (literally, “salt-loving”), whereas xerophilic

 3. Oxidation–reduction potential (Eh)
The reducing and oxidizing power of the food will influence the type of organism and chemical changes produced in the food. The concentration of oxygen in food, chemical composition and type of microorganisms associated contribute to the oxidation-reduction (O-R) potential of food and affect growth of microorganisms in them. The O-R potential of a food may be defined as the ease with which the substrate loses or gains electrons.
The Redox potential of food is determined by characters such as:
(a) Oxygen tension of atmosphere above the food,
(b) Access of atmosphere to the food,
(c) Resistance of food to the changes occurring and
(d) O-R state of materials present in food.
On the basis of the ability of microorganism to utilize oxygen, organisms are classified as aerobic, anaerobic and facultative anaerobes. Aerobes require free oxygen and anaerobes don’t prefer oxygen as it is toxic to them, hence, it is grow in the absence of molecular oxygen. Facultative may grow both aerobic and anaerobic conditions.
Generally fungi- mould and yeasts are aerobic. But bacteria are variables of these aspects. Some are aerobic, some are anaerobics and others are facultative anaerobes. If oxidation potential is high then aerobes will grow better than anaerobes, but if conditions become more reduced then anaerobes will be the predominant organisms.The O-R potential is written as Eh and measured and expressed as millivolts (mV). If the substrate is highly oxidized would have a positive Eh and substrate is reduced is a negative Eh. Aerobic microorganisms such as bacilli, cocci, micrococci, pseudomonas, acinetobacters require and grow at positive O-R potential and anaerobe such as Clostridia and bacteriodes require negative O-R potential for their growth.
Most of the fresh plant and animal food have low redox potential because of reducing substances present in them. Fresh vegetables and fruits contain reducing substances such as ascorbic acid, reducing sugars and animal tissues have sulfhydryl (-SH) and other reducing group compounds considered as antioxidants.

Fresh vegetables, fruits and meat promote growth of aerobic microorganisms in the surface regions because of positive redox potential. However, the anaerobic microorganisms grow in inner parts of vegetables, fruits and meat because of negative redox potential. Most of processed plant and animal food gain positive redox potential therefore promote growth of aerobic organisms.

4.Nutrient Content
In order to grow and function normally, the microorganisms of importance in foods require the
1. water
2. source of energy
3. source of nitrogen
4. vitamins and related growth factors
5. minerals
With respect to the other four groups of substances, molds have the lowest requirement,followed by Gram-negative bacteria, yeasts, and Gram-positive bacteria.As sources of energy, foodborne microorganisms may utilize sugars, alcohols, and amino acids.Some microorganisms are able to utilize complex carbohydrates such as starches and cellulose as sources of energy by first degrading these compounds to simple sugars. Fats are also used by microorganisms as sources of energy, but these compounds are attacked by a relatively small number of microbes in foods. The primary nitrogen sources utilized by heterotrophic microorganisms are amino acids. A large number of other nitrogenous compounds may serve this function for various types of organisms.

Some microbes, for example, are able to utilize nucleotides and free amino acids, whereas others are able to utilize peptides and proteins. In general, simple compounds such as amino acids will be utilized by almost all organisms before any attack is made on the more complex compounds such as high-molecular-weight proteins. The same is true of polysaccharides and fats. Microorganisms may require B vitamins in low quantities, and almost all natural foods have an abundant quantity for those organisms that are unable to synthesize their essential requirements. In general, Gram-positive bacteria are the least synthetic and must therefore be supplied with one or more of these compounds before they will grow. The Gram-negative bacteria and molds are able to synthesize most or all of their requirements. Consequently, these two groups of organisms may be found growing on foods low in B vitamins. Fruits tend to be lower in B vitamins than meats, and this fact, along with the usual low pH and positive Eh of fruits, helps to explain the usual spoilage of these products by molds rather than bacteria.

Antimicrobial Constituents
The stability of some foods against attack by microorganisms is due to the presence of certain naturally occurring substances that possess and express antimicrobial activity. Some plant species are known to contain essential oils that possess antimicrobial activity. Among these are eugenol in cloves, allicin in garlic, cinnamic aldehyde and eugenol in cinnamon, allyl isothiocyanate in mustard, eugenol and thymol in sage, and carvacrol (isothymol) and thymol in oregano.47 Cow’s milk contains several antimicrobial substances, including lactoferrin (see below), conglutinin, and the lactoperoxidase system (see below). Raw milk has been reported to contain a rotavirus inhibitor that can inhibit up to 106 pfu (plaqueforming units)/ml. It is destroyed by pasteurization. Milk casein as well as some free fatty acids have been shown to be antimicrobial under certain conditions.
Eggs contain lysozyme, as does milk, and this enzyme, along with conalbumin, provides fresh eggs with a fairly efficient antimicrobial system. The hydroxycinnamic acid derivatives (p-coumaric, ferulic, caffeic, and chlorogenic acids) found in fruits, vegetables, tea, molasses, and other plant sources all show antibacterial and some antifungal activity. Lactoferrin is an iron-binding glycoprotein that is inhibitory to a number of foodborne bacteria and its use as a microbial blocking agent on beef carcasses. Ovotransferrin appears to be the inhibitory substance in raw egg white that inhibits Salmonella enteritidis.

5.Biological Structures
The natural covering of some foods provides excellent protection against the entry and subsequent damage by spoilage organisms. In this category are such structures as the testa of seeds, the outer covering of fruits, the shell of nuts, the hide of animals, and the shells of eggs. In the case of nuts such as pecans and walnuts, the shell or covering is sufficient to prevent the entry of all organisms. Once cracked, of course, nutmeats are subject to spoilage by molds. The outer shell and membranes of eggs, if intact, prevent the entry of nearly all microorganisms when stored under the proper conditions of humidity and temperature. Fruits and vegetables with damaged covering undergo spoilage much faster than those not damaged. The skin covering of fish and meats such as beef and pork prevents the contamination and spoilage of these foods, partly because it tends to dry out faster than freshly cut surfaces. Taken together, these six intrinsic parameters represent nature’s way of preserving plant and animal tissues from microorganisms. By determining the extent to which each exists in a given food, one can predict the general types of microorganisms that are likely to grow and, consequently, the overall stability of this particular food. Their determination may also aid one in determining age, and possibly the handling history of a given food.

The extrinsic parameters of foods are not substrate dependent. They are those properties of the storage environment that affect both the foods and their microorganisms. Those of greatest importance to the welfare of foodborne organisms are as follows:
1. temperature of storage
2. relative humidity of environment
3. presence and concentration of gases
4. presence and activities of other microorganisms

Temperature of Storage
Microorganisms, individually and as a group, grow over a very wide range of temperatures. Therefore, it is well to consider at this point the temperature growth ranges for organisms of importance in foods as an aid in selecting the proper temperature for the storage of different types of foods. The lowest temperature at which a microorganism has been reported to grow is 34C; the highest is somewhere in excess of 100C. It is customary to place microorganisms into three groups based on their temperature requirements for growth. Those organisms that grow well at or below 7C and have their optimum between 20C and 30C are referred to as psychrotrophs . Those that grow well between 20C and 45C with optima between 30C and 40C are referred to as mesophiles, whereas those that grow well at and above 45C with optima between 55C and 65C are referred to
as thermophiles

Relative Humidity of Environment
The RH of the storage environment is important both from the standpoint of aw within foods and the growth of microorganisms at the surfaces. When the aw of a food is set at 0.60, it is important that this food be stored under conditions of RH that do not allow the food to pick up moisture from the air and thereby increase its own surface and subsurface aw to a point where microbial growth can occur.
When foods with low aw values are placed in environments of high RH, the foods pick up moisture until equilibrium has been established. Likewise, foods with a high aw lose moisture when placed in an environment of low RH. There is a relationship between RH and temperature that should be borne in mind in selecting proper storage environments for foods. In general, the higher the temperature, the
lower the RH, and vice versa. Foods that undergo surface spoilage from molds, yeasts, and certain bacteria should be stored under conditions of low RH. Improperly wrapped meats such as whole chickens and beef cuts tend to suffer
much surface spoilage in the refrigerator before deep spoilage occurs, due to the generally high RH of the refrigerator and the fact that the meat-spoilage biota is essentially aerobic in nature. Although it is possible to lessen the chances of surface spoilage in certain foods by storing under low conditions of RH, it should be remembered that the food itself will lose moisture to the atmosphere under such
conditions and thereby become undesirable. In selecting the proper environmental conditions of RH, consideration must be given to both the possibility of surface growth and the desirable quality to be maintained in the foods in question. By altering the gaseous atmosphere, it is possible to retard surface spoilage without lowering the RH.

Presence and Concentration of Gases in the Environment
Carbon dioxide (CO2) is the single most important atmospheric gas that is used to control microorganisms in foods.It along with O2 are the two most important gases in modified atmosphere packaged (MAP) foods, Ozone (O3) is the other atmospheric gas that has antimicrobial properties, and it has been tried over a number of decades as an agent to extend the shelf life of certain foods. It has been shown to be effective against a variety of microorganisms,9 but because it is a strong oxidizing agent, it should not be used on high-lipid-content foods since it would cause an increase in rancidity. Ozone was tested against Escherichia coli 0157:H7 in culture media, and at 3 to 18 ppm the bacterium was destroyed in 20 to 50 minutes.10 The gas was administered from an ozone generator and on tryptic soy agar, the D value for 18 ppm was 1.18 minutes, but in phosphate buffer, the D value was 3.18 minutes. To achieve a 99% inactivation of about 10,000 cysts of Giardia lamblia per milliliter, the average concentration time was found to be 0.17 and 0.53 mg-min/L at 25C and 5C, respectively.53 The protozoan was about three times more sensitive to O3 at 25C than at 5C. It is allowed in foods in Australia, France, and Japan; and in 1997 it was accorded GRAS (generally regarded as safe) status in the United States for food use. Overall, O3 levels of 0.15 to 5.00 ppm in air have been shown to inhibit the growth of some spoilage bacteria as well as yeasts
Presence and Activities of Other Microorganisms
Some foodborne organisms produce substances that are either inhibitory or lethal to others; these include antibiotics, bacteriocins, hydrogen peroxide, and organic acids. The bacteriocins and some antibiotic

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